In the process industry, valves typically operate on a continuous basis. The life time of the valve can depend on how well the valve reaches a position specified by the control system. However, there can be several reasons why a valve might run into a cycling state as it tries to reach its destination.
Some of these cycling behaviors may damp themselves, eventually letting the valve settle to a steady state. But, regardless of whether the valve becomes stable, the cycling behavior is detrimental to both the valve and the control. Cycling causes the valve to wear out quickly, and causes the valve to take longer to meet the control system requirements resulting in a sub-optimal process. Detecting the cycling behavior and correcting it can significantly improve the valve life and the process efficiency.
Cycling behaviors can be classified into categories, which can include stick-slip cycling (also known as “stiction”), shut-off cycling, and flow loop aggressive tuning.
Stick-slip or stiction is a phenomenon in which the valve sticks due to friction from tight packaging, debris, etc. and fails to respond even as force is exerted on the valve stem. Eventually, as the force is increased to break the friction, the valve slips and has a jerky motion. This can make the process inefficient as the valve becomes less responsive. Furthermore, depending on the cause of this behavior, it can also lead to increased wear on the valve.
Shut-off cycling is a phenomenon in which the valve frequently shuts off and opens in a cyclic manner. Valves generally have a shut-off limit. When the valve reaches that position it needs to shut off completely, cutting off any flow through the valve. Over time, however, the valve can develop a loose linkage, sensor drift, or an incorrectly-set shut-off limit. These issues can cause the valve to operate close to the shut-off limit, causing the valve to shut off and open frequently. Since shut-off is hard on the plug, frequent shut-off can damage the valve.
Flow loop aggressive tuning is a phenomenon in which the control system does not wait long enough for the valve to catch up, causing the valve to cycle. In other words, the valve continuously tries to catch up with the set-point, which is changed aggressively by the control system to compensate for the error. This again can wear out the valve due to unneeded and often continuous movement of the valve.
In the past, methods to detect slick-slip have depended on either the velocity of the valve plug or the shape of a graph of valve position versus set-point. The velocity-based approach is not very efficient when there is lack of motion due to the valve sticking or due to the valve being held at a steady state. It is thus important to look at the set-point to understand whether the valve is expected to move. The other approach, which compares the set-point and the valve position, attempts to do this. Detection based on the shape of this plot, however, can be especially inaccurate in the presence of noise, which can deform the shape of the graph. Existing systems do not have provisions for detecting other cycling behaviors, such as shut-off cycling and flow loop aggressive tuning.
Accordingly, a need exists for systems and methods for diagnosing components of a control system.